Modulator



A g- 1951 D. F. WINTER 2,554,045

MODULATOR Filed Jan. 7, 1946 F|G.I

7 PuLsE FORMING NETWORK H Q .0 l .5

TIME r INVENTOR DAVID F. WINTER VOLTAGE B) TIME W W- Q/ zzi ATTORNEY Patented Aug. 14, 1951 MODULATOR David F. Winter, Cambridge, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application January 7, 1946, Serial No. 639,639

4 Claims.

My invention relates in general to electrical circuits and more particularly to modulators for pulsed radio oscillators.

In the progression of the radio frequency spectrum toward shorter and shorter wavelengths, the use of magnetron oscillators has become increasingly great, by reason of the ability of such tubes to produce high power outputs at extremely short wavelengths. However, the theory and practice of magnetron techniques have not reached a stage of development where the behavior of such an oscillator in pulsed operation is completely predictable. Hence it is often necessary to study the effects of various circuit parameters on new oscillators.

A common modulator circuit for pulse production consists of a network or artificial line which is charged from a high voltage source and discharged abruptly to generate a short, substantially rectangular voltage pulse which furnishes plate voltage for a magnetron tube.

The duration and shape of the pulse depends upon the lumped reactances of which the network is composed Since it is impractical to adjust the individual reactances in order to vary the output pulse and inconvenient to design several interchangeable networks, it is clear that a device which obviates these two adjustments is of considerable value.

Accordingly, it is one object of my invention to provide means for varying certain parameters in a magnetron oscillator circuit.

Another object of my invention is to provide a simple means of altering the shape of the output of a pulse-forming network.

Still another object is to provide an improved test circuit for magnetron tubes.

Briefly, my invention accomplishes these and further objects which will be apparent to those skilled in the art by the connection of a variable time-constant series resistance-capacitance circuit across the output of the pulse-forming network. This branch also contains a biased diode which isolates the branch from the network until the pulse voltage is enough to overcome the bias on the diode. Hence the rate of voltage increase applied to the oscillator is changed when the additional load of the resistance-capacitance circuit is put in parallel with the oscillator. For this reason this circuit in my invention will be termed hereinafter as a rate-changer.

The rinciples and operation of my invention will be more apparent upon reference to the following specification, claims, and to the drawings, in which:

Fig. 1 represents a test circuit employin one embodiment of my invention;

Fig. 2 shows typical variations in the shape of the modulating pulse with different types of load; and

Fig. 3 shows variations in the shape of the modulating pulse occasioned by alteration of the t me constant of the rate-changer.

The circuit of Fig. 1, to which figure reference is now made, is a simplified form of testing device for new magnetrons. A voltage source with the polarity indicated is connected between terminals 5. The more positive terminal is connected through inductor 6 to pulse-forming network "I. Between the input to network I and the more negative of terminals 5 is connected 2. short-circuiting switch 9. To the output of network 1 is connected rate-changer [0 which consists of the series circuit enclosed by dotted lines. The individual elements of rate-changer it are diode II, the cathode of which is tied to the output of network I, variable resistor I2, capacitor [3, and variable bias source Hi, all four elements bein connected in series. The more positive terminal of bias source [4 is connected to the more negative of terminals 5. In parallel with capacitor I3 is bleeder resistor IS. The output of network I is also applied to the center conductor of coaxial line H, the outer conductor of which is maintained at the potential of the more negative of terminals 5 by connection thereto. Primary winding 18 of a pulse transformer is the termination of line l1 and the voltages developed across secondary winding I9 are impressed on magnetron tube 20.

The operation of the apparatus of Fig. 1 can best be explained in conjunction with Fig. 2 which shows, superimposed, curves of the pulse voltage output of network I plotted as a function of time under difierent circuit conditions.

Assume that network I is operating into a resistance load equal to the characteristic impedance of the network. While switch 9 is open the capacitors of network I are charged through charging inductance 6. When switch 9 is closed at some time such as T1 the charging voltage is abruptly removed and the capacitance of network I discharges through the load impedance. If the network consisted entirely of capacitors, the discharge would be exponential. However, the presence of inductors in the network causes the output to be other than exponential, the

exact shape depending on the number, type, and connection of the reactors used. Hence the voltage at the output terminal of network I will take v.biasedanode.

a form such as that of curve A in Fig. 2. At time T2 switch 9 is open and the network recharges.

Since the pulse-forming network will actually be operating into a magnetron which may have an internal resistance of some 800 ohms instead of a matched resistance of about 50 ohms, a pulse transformer is used to change the impedance level without altering the pulse shape. Therefore network I will actually be discharging through primary winding 18. Line ll, having a characteristic impedance substantially equivalent to that of the pulse source, does not afiect the impedances and allows the oscillator to be physically separat- 1 ed from the modulator.

Even though the impedances are matched by means of the transformer as carefull as possible, the reactances introduced into the circuit and the non-linear characteristic of the oscillator load will affect the outputof the pulse-forming net- .work. The net result will be a somewhat irregular pulse such as is shown in curve B of Fig. 2. This -pulse shape is undesirable.

.The result of adding rate-changer I is shown in curve C. The potential from bias source [4 is adjustedto a predetermined level, a possible value being indicated by the dotted line. 'Diode II is therefore non-conducting until the pulse voltage has-.drivenits cathode potential below that of the Hence curve C follows curve B closely for a time. When diode l 1 begins to conduct, capacitor it can charge through resistor [2, loading the circuit and damping out the oscillations which characterize curveB. Curve C there- .fore shows a substantially exponential drop until .capacitor 13 is charged, after which curve C con- .iorms. withcurve A, since, the rate-changer no longerdraws current. Attime T2 the cathode of ,,.diode Hagainbecomes more positive than the ,anode becauseof the. charge on capacitor l3. Hence diode H-cannot conduct, the rate-changer =isefiective1y. disconnected from. network 7, and "the trailing edge of this pulse is unafiected. Ca- ,pacitorls discharges through bleeder resistor I .andthe circuitv is prepared for another cycle.

The control over the pulse rise rate exercised .by resistor i2 is illustrated in Fig. 3. Curve A in vthis figure is the same as the similarly lettered .curve, in. Fig. 2, except that it is expanded on the voltage scale. Curve D represents a low, curve E .amediamand curve F ahigh resistance setting of resistor l2. It. can be clearly seen that once the ,bias level has been reached a high degree of control over the shape of the pulse front may be exercised. In combination with the variability of ..the bias level, this control affords ,an almost infinite variety of pulse shapes.

..Although my invention has been presented as applicable to experimental and test apparatus, it

will be understood that it is by no means limited thereto. It will further be apparent .to those .skilled-in the art that the embodiment which has .been shown anddescribed may be modified without departing from the principles of my invention.

'4 Hence I claim all such modifications and adaptations as may fall fairly within the spirit and scope of the hereinafter appended claims.

What is claimed is:

1. In combination with a pulse generating apparatus having a pulse-forming network, means for storing electrical energy in said pulse-forming network, load means coupled to one end of said pulse-forming network, and means for intermittently discharging the energy in said pulse-forming network through said load means; a pulseshaping circuit including a time constant circuit, saidpulse-shaping circuit being connected substantially in parallel with said load means for controlling the discharge rate of said pulse-forming network through said load means.

2. The combination according to claim 1,

'wherein said pulse-shaping circuit further includes a unilaterally conducting device in series with said time constant circuit, and bias means for rendering said unilaterally conducting device non-conducting until the potential difierence across said pulse-shaping circuit exceeds apredetermined magnitude.

3. The combination according to claim 2, wherein said time constant circuit includes afirst resistance, a capacitance in series with said first resistancaand a second bleeder resistance shunting said capacitance.

4. A pulse generating apparatuscomprising a ulse-forming network; first and second impedances; a load coupled to said second "impedance;

a pulse shaping circuit shunting said second'impedance, said pulse shaping circuit including a time constant circuit, a unilaterally conducting 'device in series with saidtime constant "circuit,

and bias means for rendering said-unilaterally conducting device non-conducting until the .po-

" tential difference across said pulse shapingc'ircuit exceeds a predetermined magnitude; means to the other end of said pulse-forming network; means for applying a direct currentvoltagebetween the other ends of said first and second impedances for storing energy in said pulse-forming network; and means for intermittently connecting said one end of said pulse-formingnetwork to said other end of said second impedance for discharging said pulse-forming network'through said load, said second impedance; and said pulseshaping circuit.

DAVID TEE.

REFERENCES CITED The following. references areof record-in the Aim of this-patent:

' UNITED STATES T PATENTS Number Name Date 2,405,070 Tonks et al July 30', 1946 2,417,834 Lord -r Mar. 25', 31947 

